Pernambuco state fire department spokesman Marcio Maia said 14-year-old Wellington dos Santos was attacked on Sunday after he swam beyond a coral reef that keeps sharks away from the beach of Piedade near the state capital, Recife.
Lifeguards rescued dos Santos and rushed him to a hospital, where he was reported to be in critical but stable condition, Maia said.
Besides severing dos Santos' hand, the shark "bit off a large chunk of his buttocks," Maia said.
"People insist on ignoring the sign posts warning of the danger of shark attacks, especially beyond the coral reefs about 150 meters (490 feet) from the beach," Maia said.
The attack was the 51st since authorities started keeping count of shark attacks in the area in 1992. Maia said sharks have killed 19 people in Pernambuco state over the past 15 years.
